ARTICLES
PUBLISHED ONLINE: 7 SEPTEMBER 2015 | DOI: 10.1038/NNANO.2015.194
A phosphorene–graphene hybrid material as a
high-capacity anode for sodium-ion batteries
Jie Sun1†, Hyun-Wook Lee1†, Mauro Pasta1, Hongtao Yuan2,3, Guangyuan Zheng4, Yongming Sun1,
Yuzhang Li1 and Yi Cui1,3*
Sodium-ion batteries have recently attracted significant attention as an alternative to lithium-ion batteries because sodium
sources do not present the geopolitical issues that lithium sources might. Although recent reports on cathode materials
for sodium-ion batteries have demonstrated performances comparable to their lithium-ion counterparts, the major
scientific challenge for a competitive sodium-ion battery technology is to develop viable anode materials. Here we show
that a hybrid material made out of a few phosphorene layers sandwiched between graphene layers shows a specific
capacity of 2,440 mA h g−1 (calculated using the mass of phosphorus only) at a current density of 0.05 A g−1 and an 83%
capacity retention after 100 cycles while operating between 0 and 1.5 V. Using in situ transmission electron microscopy
and ex situ X-ray diffraction techniques, we explain the large capacity of our anode through a dual mechanism of
intercalation of sodium ions along the x axis of the phosphorene layers followed by the formation of a Na3P alloy. The
presence of graphene layers in the hybrid material works as a mechanical backbone and an electrical highway, ensuring
that a suitable elastic buffer space accommodates the anisotropic expansion of phosphorene layers along the y and z axial
directions for stable cycling operation.
S
odium-ion batteries have received a great deal of attention
lately as an attractive alternative to lithium-ion batteries,
because sodium resources are virtually inexhaustible and are
ubiquitous around the world1. Recent studies on sodium-ion
battery cathode materials have reported performances comparable
to their lithium-ion battery counterparts2–4. The major scientific
challenge for sodium-ion batteries therefore resides in developing
a new anode material with a high specific capacity and an appropriately low redox potential. Adapting lithium-ion battery anode
materials to sodium-ion batteries has so far been unsuccessful.
Metallic sodium is not a suitable material because of dendrite formation and its low melting point (98 °C)5, and graphite is electrochemically inactive due to the large size of the sodium ions (2.04 Å)
compared with the channel size of the graphene layers (1.86 Å)6.
It has recently been shown that the graphene interlayer distance
could be increased to 4.3 Å and the sodium ions inserted reversibly,
with promising capacity retention7. However, only a modest specific
capacity of 284 mA h g−1, insufficient for applications, was
measured. Other carbon anodes, for example composed of hard
carbon or amorphous carbon, show specific capacities of less than
300 mA h g−1 5. Silicon, despite the theoretical existence of Na–Si
alloys, is also electrochemically inactive8. Other group IV elements
such as germanium (Na15Ge4 , 369 mA h g−1)9,10, tin (Na15Sn4 ,
847 mA h g−1)11,12 or lead (Na15Pb4 , 485 mA h g−1)13, and group V
elements such as antimony (Na3Sb, 660 mA h g−1)14, have
been suggested as promising anode candidates, but they exhibit
specific capacities below 1,000 mA h g−1 and limited cycle life
(Supplementary Table 1).
Phosphorus reacts electrochemically with both lithium and
sodium to form Li3P and Na3P at an attractive potential for an
anode material and with a high theoretical specific capacity of
2,596 mA h g−1, which significantly exceeds that of any other
sodium-ion battery anode presently available. Phosphorus exists in
three main allotropes: white, red and black15. White phosphorus consists of tetrahedral P4 molecules. It is highly reactive and toxic and
therefore unsuitable as a battery material16. Amorphous red phosphorus has been investigated previously as an anode material for
both lithium-ion17–19 and sodium-ion batteries20–23, but its poor electrical conductivity results in a modest cycle life and rate performance.
Orthorhombic black phosphorus with its layered crystal structure is
the most thermodynamically stable allotrope24,25. In terms of appearance, properties and structure, black phosphorus closely resembles
graphite: it is black and flaky, a good conductor of electricity
(∼300 S m−1), and it is made of puckered sheets of covalently
bonded phosphorus atoms24,25. Compared with graphite, black phosphorus has a larger interlayer channel size (3.08 versus 1.86 Å),
meaning that both lithium (1.52 Å) and sodium (2.04 Å) ions can
be stored between the phosphorene layers. Although black phosphorus has been reported previously in the literature as an anode
for lithium-ion batteries26–29, its poor cycling performance for
sodium-ion batteries needs to be improved30. It is therefore important
to investigate the sodiation mechanism in order to optimize the
performance of black phosphorus for use in sodium-ion batteries.
In this Article, we investigate the sodiation mechanism of black
phosphorus using in situ transmission electron microscopy
(TEM) and ex situ X-ray diffraction (XRD) techniques. A twostep sodiation mechanism of intercalation and alloying is demonstrated. A large anisotropic volumetric expansion along the y and
z axial directions is observed during the alloy reaction, which
leads to a significant capacity decay with cycling. To achieve both
high capacity and stable cyclability, we have specifically designed a
nanostructured phosphorene–graphene hybrid with a few phosphorene layers sandwiched between graphene layers. The advantages of our structure design are threefold: (1) the graphene layers
1
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA. 2 Geballe Laboratory for Advanced Materials,
Stanford University, Stanford, California 94305, USA. 3 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory,
2575 Sand Hill Road, Menlo Park, California 94025, USA. 4 Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA.
†These authors contributed equally to this work. e-mail: [email protected]
*
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
1
ARTICLES
a
NATURE NANOTECHNOLOGY
Black phosphorus
Intercalated black phosphorus
DOI: 10.1038/NNANO.2015.194
Na3P
Along x axis
gx
Intercalation
d1
Alloying
gy
d2
High reactivity
z
d1 = 2.224 Å
d2 = 2.244 Å
Along x direction (gx) = 3.08 Å
Along y direction (gy) = 1.16 Å
y
x
b
c
2.1
d
80 s
Sodium
e
170 s
Å
xa
d=
0s
Phosphorus
n
xi s
sio
an
%
92
x
y
2 nm
f
p
ex
0.2 µm
0.2 µm
Na
0.2 µm
Graphene
Graphene
Sodiation
Desodiation
Na3P
Phosphorene
Figure 1 | Mechanism of sodiation in black phosphorus. a, Schematics of black phosphorus before sodiation, with the first step of sodium-ion intercalation,
and the second step of alloy reaction to form Na3P. b, High-resolution and bright-field TEM image of black phosphorus before sodiation. c–e, Time-lapse
TEM images of sodiation in black phosphorus. Sodium ions transport along the x-axis channel, causing a volume expansion along the y-axis direction.
f, Structural evolution of the sandwiched phosphorene–graphene structure during sodiation.
provide an elastic buffer layer to accommodate the anisotropic volumetric expansion during the sodiation process; (2) the phosphorene
layers offer a short diffusion distance for sodium ions; (3) the graphene layers function as an electrical highway, so the hybrid
material is electrochemically active.
Mechanisms of sodiation in black phosphorus
The storage of sodium ions in black phosphorus proceeds according
to two different reaction mechanisms of intercalation and alloying,
as shown in Fig. 1a. Sodium ions are first inserted between the phosphorene layers (intercalation), along the x-axis-oriented channels.
Indeed, only the channels along the x axis are wide enough (3.08 Å)
to allow the diffusion of sodium ions (2.04 Å), the channels along
the y axis being too small at 1.16 Å. An ex situ XRD diffractogram
of black phosphorus before sodiation (Supplementary Fig. 2)
show two characteristic diffraction peaks at 16.9° (d = 5.24 Å) and
34.2° (d = 2.62 Å), attributed to the diffraction of the (002) and
(004) crystal planes, respectively. These two peaks shift towards a
lower angle with the sodiation of black phosphorus, indicating
that the distance between the phosphorene layers along the z axis
has increased and revealing the intercalation of sodium ions. The
two XRD peaks are clearly visible until the potential reaches 0.54 V
(Supplementary Fig. 2b,c), clearly showing that the layered structure
is preserved upon cycling. This mechanism is similar to the intercalation process occurring in graphite upon lithiation, generating
the LiC6 phase31. The intercalation does not significantly alter the
host structure, thus guaranteeing high reversibility upon cycling in
the 0.54–1.5 V region (Supplementary Fig. 3a). Nonetheless, the
specific capacity is limited to the impractical value of 150 mA h g−1,
that is, Na0.17P.
2
Further sodiation below 0.54 V results in the formation of NaxP
species (alloy reaction), with the consequent disappearance of the
characteristic black phosphorus XRD peaks (Supplementary Fig. 2).
Diffraction peaks of the intermediate NaxP phases were difficult to
identify due to domain size, poor crystallinity and the Kapton tape
background used to encapsulate the air-sensitive electrodes.
However, when the potential reaches 0.1 V and the alloy reaction is
close to completion, the distinctive (110) and (103) peaks of the
Na3P phase are clearly recognized at 36.0° (d = 2.49 Å) and 37.0°
(d = 2.42 Å) (Supplementary Fig. 2a). The alloying mechanism is
therefore primarily responsible for the outstanding specific capacity
of black phosphorus (Supplementary Fig. 3c). However, this mechanism also results in a large volume expansion of ∼500%, which causes
mechanical and structural fracturing (Supplementary Fig. 4), leading
to significant capacity loss upon cycling (Supplementary Fig. 3d).
These challenges are similar to those with other high-capacity
materials such as silicon32,33 and sulphur34.
It is therefore necessary to understand the nature of the stress
evolution in black phosphorus in order to efficiently tackle these
problems by developing innovative materials structures. Figure 1b,c
presents TEM images of a black phosphorus particle before in situ
sodiation. The longer, clean-cut edge of the particle is aligned
along the x-axis direction. In fact, cutting along the x axis requires
only four P–P bonds to be broken, which are longer (2.244 Å)
and therefore weaker (purple dashed line in Figs 1a and 2d).
It would be impossible to obtain such a clean-cut edge along the
y-axis direction given that the shorter (2.224 Å) P–P bond would
still hold the phosphorene layers together. Moreover, we observed
many nanobelt-like objects among the general nanosheet structures
(Supplementary Figs 6 and 7). All the nanobelts are also clean-cut
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
NATURE NANOTECHNOLOGY
a
ARTICLES
DOI: 10.1038/NNANO.2015.194
c
b
d
d = 7.7 Å
d1 = 2.224 Å
d2 = 2.244 Å
d = 4.4 Å
5 nm
2 layers
x
1 layer
x
y
1 nm
f
4
3’
2
2’
5’
(nm)
4’
3
1
1’
5
High reactivity
g
2
1
0
1
0
1
0
4
2
0
4
2
0
1.57 nm
1 1’
0.84 nm
2 2’
1.53 nm
3 3’
2.34 nm
3.21 nm
0
2
4
4.18 nm
4 4’
4.22 nm
5 5’
6 8
(µm)
10
12
h
(004)
(002)
Intensity (a.u.)
e
y
Bulk phosphorus
d = 5.2 Å
d = 7.7 Å
(020) (006)
(012)
Phosphorene
10
20
30
40
50
60
8
12 16 20
2θ (deg)
Figure 2 | Evidence of monolayer and few-layer phosphorene. a, TEM image of monolayer phosphorene. Scale bar, 2 μm. b, TEM image of trilayer
phosphorene nanobelt showing the long side. Scale bar, 200 nm. Inset: HRTEM image of the edge. c, HRTEM image of selected area of b with measured
lattice spacing. d, Schematic of square phosphorene (top view) with labelled bond lengths. e, AFM image of monolayer and few-layer phosphorene.
Scale bar, 2 μm. f, Measured thickness of the phosphorene in e. g, XRD pattern of black phosphorus and phosphorene. h, Amplification of XRD pattern of
black phosphorus and phosphorene in the low-angle range (dashed rectangle in g).
along their longer sides (Fig. 2b), and always aligned along the x
axis, according to the high-resolution (HR) TEM image in Fig. 2c.
The potentiostatic sodiation of the black phosphorus particle
(Fig. 1c) occurred as it was separated from the sodium metal electrode by a native sodium oxide layer acting as a solid electrolyte
(Supplementary Section 1.3, ‘In situ TEM’). Figure 1d,e shows the
black phosphorus particle after 80 and 170 s (Supplementary
Movie 1), respectively. A 92% expansion along the y axis was
observed, while the particle dimension along the x axis remained
nearly unchanged. The anisotropic expansion along the y axis is
the result of easier sodium ion diffusion along the x axis and the
weaker P–P bonding along the x axis. The full sodiation in black
phosphorus forming Na3P has a theoretical volume expansion of
∼500% and the x and y axis expansions have been measured to be
0 and 92%, respectively. From this, we infer that a significant expansion (∼160%) along the z axis is also taking place. Therefore, minimizing the width and the thickness of the black phosphorus layers
would help reduce the stress built up along the critical y and z
axial directions. In this sense, an electrode made of monolayer
black phosphorus (phosphorene)35–38 would be ideal. In broad
terms, few-layer (<10 layers) black phosphorus can be defined as
phosphorene, much the same as graphene39,40. Moreover, sandwiching small phosphorene layers between large graphene sheets
(Fig. 1f ) could lead to the following additional benefits: (1) the ultrathin phosphorene minimizes the diffusion distance of both sodium
ions and electrons, resulting in an enhanced rate performance; (2)
the graphene layers can facilitate the transport of electrons generated
in the phosphorene redox reaction to the current collector; (3) the
space between phosphorene nanosheets provides an elastic buffer
space to accommodate anisotropic expansion.
Fabrication of phosphorene–graphene hybrids
Few-layer phosphorene nanosheets were exfoliated using a technique similar to a previously reported method41,42 that is versatile
and solution-based, making it relatively scalable, and with yields
approaching 17 wt% (see Methods). The as-synthesized few-layer
phosphorene nanosheets were imaged by TEM (Fig. 2a,b) and
atomic force microscopy (AFM) (Fig. 2e,f ). Their thicknesses
ranged between 0.84 and 4.22 nm, equivalent to one to five phosphorene layers (rarely did the thickness exceed five layers). The
width of each nanosheet ranged between 700 nm and 10 μm. The
HRTEM image in the inset to Fig. 2b shows a nanosheet made of
three phosphorene layers with a [002] lattice spacing of 7.7 Å,
which is larger than the 5.2 Å of black phosphorus, as confirmed
by an XRD diffractogram (Fig. 2g,h). Thus, the x and y channel
sizes increase to 3.7 and 5.6 Å, respectively, opening the y channel
to sodium ion (2.04 Å) diffusion.
The structural differences between black phosphorus (>10 layers)
and the phosphorene nanosheets (1–5 layers) were further investigated by Raman spectroscopy (Supplementary Fig. 5a). Atomic
displacements of Raman active modes corresponding to one
out-of-plane mode (A1g) and two in-plane modes (A2g and B2g)43–45
are shown in Supplementary Fig. 5b. We observed A1g , A2g and B2g
modes in both black phosphorus and phosphorene samples, with
the ratios A2g/A1g and A2g/B2g increasing as the number of phosphorene layers decreased. For mono- and bilayer phosphorene, the A2g
mode shifted from 467.7 cm−1 (3–5 layers) to 469.6 and 468.8 cm−1,
respectively. Black phosphorus is a unique anisotropic material
with the lattice parameters in different directions exhibiting different sensitivities to external impacts, so the in-plane mode A2g
vibration stiffens with a decrease in thickness, while the B2g and
A1g modes remain almost unchanged46.
The phosphorene–graphene sandwich structure was achieved
simply by mixing N-methyl-2-pyrrolidone (NMP) dispersions of
phosphorene and graphene (Fig. 3a), followed by self-assembly
after NMP evaporation in an argon-filled glove box (see
Methods). TEM images with energy-dispersive X-ray spectroscopy
(EDS) mapping (Supplementary Fig. 8) show phosphorene
nanosheets uniformly distributed and sandwiched between the
wider graphene nanosheets. The HRTEM image of the crosssection reported in Fig. 3c shows a thickness of 2–5 nm, with alternating phosphorene and graphene layers. Raman spectroscopy
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
3
ARTICLES
a Graphene
NATURE NANOTECHNOLOGY
Phosphorene
Phosphorene + graphene
DOI: 10.1038/NNANO.2015.194
the poor affinity between the phosphorene layers, due to their
buckled structure. The X-ray photoemission spectroscopy (XPS)
peaks at the P2p and C1s edges are not significantly affected by
the self-assembly process (Fig. 3e). This clearly indicates that no
chemical reaction occurs during the synthesis and that the structure
is held together by van der Waals forces.
Electrochemical testing of phosphorene–graphene hybrids
b
c
P
G
G
P
G
d
Intensity (a.u.)
G
2D
Ag2
Ag1
B2g
D
500
1,000
1,500
2,000
2,500
3,000
Raman shift (cm−1)
e
129.9 eV
P/G
284.6 eV
P/G
G
P
133
C1s
Intensity (a.u.)
Intensity (a.u.)
P2p
132
131
130 129
Binding energy (eV)
128 290 288 286 284 282 280
Binding energy (eV)
Figure 3 | Physical characterization of the phosphorene–graphene hybrid
structure. a, Digital photographs of NMP dispersions of graphene (3 μg ml–1),
phosphorene (17 μg ml–1) and a mixture prepared by mixing the
dispersions of graphene and phosphorene at a volume ratio of 6:1,
corresponding to a carbon–phosphorus molar ratio of 2.78:1. Red light
beams were incident from the side to demonstrate the Tyndall effect.
b, TEM image of the phosphorene–graphene hybrid. Scale bar, 2 μm.
c, HRTEM image of the cross-section of the phosphorene–graphene hybrid
(the turned-up right edge in b). Scale bar, 2 nm. d, Raman spectra of
phosphorene–graphene hybrid with the modes of both phosphorene (A1g , A2g
and B2g) and graphene (G and 2D). e, XPS analysis of the C1s and P2p
peaks from graphene, phosphorene and phosphorene–graphene. Graphene,
phosphorene and the phosphorene–graphene hybrid are labelled G, P and
P/G, respectively, in c and e.
(Fig. 3d) shows the A1g , A2g and B2g modes of phosphorene, as well as
the distinct G and 2D modes characteristic of graphene39,40. The
position of the A2g peak and the intensity of the ratios A2g/A1g and
A2g/B2g correspond to nanosheets made of five phosphorene layers
or less46. Phosphorene–phosphorene self-assembly is hindered by
4
The carbon–phosphorus (C/P) mole ratio in the hybrid structure is
an important parameter that requires further optimization. In fact,
some buffer space between different phosphorene nanoplates
should be reserved for the y-axis directional expansion of phosphorene upon sodiation. The C/P ratio was calculated based on the area
ratio of monolayer graphene and phosphorene (Supplementary
Section 2.4, ‘Phosphorene/graphene hybrid’). Considering that the
real nanosheets are made of more than one monolayer, we experimentally investigated the cycling performance of the phosphorene–graphene hybrids with various C/P ratios. The galvanostatic
cycling at 0.02 C (0.05 A g−1) (Fig. 4a) shows that samples prepared
using C/P ratios of 2.78:1 and 3.46:1, respectively, exhibit a similar
capacity retention after 100 cycles. A lower C/P ratio is obviously
beneficial in terms of specific capacity, given that graphene is
electrochemically inactive for sodiation. Therefore, the following
electrochemical characterization was carried out on the optimized
2.78:1 C/P ratio, corresponding to 48.3% of phosphorus by mass.
The electrochemical performance of the phosphorene–graphene
sandwiched structure was tested in a two-electrode arrangement, in
the 1.5–0.02 V potential range and at a 0.02 C rate (50 mA g−1)
(Fig. 4b). The first discharge cycle shows three distinct electrochemical processes. The first region between 1.2 and 0.6 V corresponds to
the irreversible formation of the solid-electrolyte interphase (SEI)
layer (Supplementary Fig. 9), which is responsible for the modest
first-cycle Coulombic efficiency of ∼80%. Two additional plateaux
are located at 0.58 and 0.2 V, corresponding to the intercalation
and alloy reaction, respectively. In subsequent cycles the 0.58 V
plateau disappears, as the phosphorene layers do not survive the
intercalation reaction. Although the phosphorene layer transformed
to an amorphous structure (Supplementary Fig. 4c) after one cycle,
it was still confined between graphene interlayers, resulting in a
stable sandwiched structure (Supplementary Fig. 11). A shorter
plateau can be seen at ∼0.4 V, which may be due to the nucleation
of a NaxP phase. The first cycle reversible capacity was 2,440 mA h g−1
based on the mass of the phosphorus. The contribution of
graphene to the reversible capacity is negligible (∼40 mA h g−1,
Supplementary Fig. 13). The theoretical specific capacity of
phosphorus is 2,596 mA h g−1 and the phosphorene–graphene
hybrid delivered 94% of the theoretical capacity (equivalent to
Na2.82P). To test the efficacy of our sandwich design, the electrochemical performance of an electrode prepared by mixing black
phosphorus and graphene with the same C/P ratio of 2.78:1 was
also measured, and a low reversible capacity of ∼700 mA h g−1
was observed (Supplementary Fig. 3c).
Due to its enhanced electrical conductivity, the phosphorene–graphene sandwiched structure showed very good cycling capability
(Fig. 4c). A reversible capacity of 2,440 mA h g–1 was obtained at
0.02 C (50 mA g−1). When the current rate was increased to 0.08 C
(0.2 A g−1), it could still deliver a reversible capacity of 2,320 mA h g–1.
Even at the very high rates of 3 C (8 A g−1), 4.6 C (12 A g−1),
7.7 C (20 A g−1) and 10 C (26 A g−1), the electrode retained a
specific capacity of 1,450 mA h g–1 (59.4%), 1,200 mA h g–1
(49.2%), 915 mA h g–1 (37.5%) and 645 mA h g–1 (26.4%), respectively. Moreover, the phosphorene–graphene sandwiched structure
shows good capacity retention upon cycling (Fig. 4d). After
100 cycles at a rate of 0.02 C (50 mA g−1), a specific capacity of
2,080 mA h g−1 was still available, corresponding to an 85% capacity
retention (0.16% decay per cycle). At relatively fast rates of 3 C (8 A g−1)
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
NATURE NANOTECHNOLOGY
ARTICLES
DOI: 10.1038/NNANO.2015.194
Volumetric capacity (mA h cm−3)
2,000
1,500
1,000
C/P = 3.46:1
C/P = 2.78:1
C/P = 2.07:1
C/P = 1.39:1
500
20
40
60
80
0.5
g −1
0.2
A g −1
0.05
A g −1
g −1
4A
g −1
g −1
8A
12 A
2,000
1.0
0.5
0
500 1,000 1,500 2,000 2,500 3,000
500
1,000 1,500 2,000 2,500
Specific capacity (mA h g−1)
3,500
100
3,000
90
2,500
0.02 C
2,000
30
1,500
3C
20
1,000
Coulombic efficiency (%)
Specific capacity (mA h g−1)
1.5
Specific capacity (mA h g−1)
Cycle number
d
1,500
0.0
0
100
1,000
g −1
g −1
1.0
0.0
0
0
1st cycle
2nd cycle
50th cycle
1.5
26 A
2,500
Potential vs. Na/Na+ (V)
3,000
500
c 2.0
50 1 2
20 A
b
Potential vs. Na/Na+ (V)
Specific capacity (mA h g−1)
a
1.6 A
0
10
500
10 C
0
0
0
10
20
30
40
50
60
70
80
90
100
Cycle number
Figure 4 | Electrochemical characterization of the phosphorene–graphene anode for sodium-ion batteries. All specific capacities of the phosphorene–
graphene anode are based on the mass of the phosphorene, because the contribution of graphene to the reversible capacity is negligible. a, Reversible
desodiation capacities for the first 100 galvanostatic cycles of various phosphorene–graphene electrodes with different carbon–phosphorus mole ratios (C/P)
of 1.39, 2.07, 2.78 and 3.46, between 0.02 and 1.5 V at a current density of 0.05 A g−1. b, Galvanostatic discharge–charge curves of the phosphorene–
graphene (48.3 wt% P) anode plotted for the first, second and 50th cycles. c, Volumetric and mass capacities at different current densities (from 0.05 to
26 A g−1). d, Reversible desodiation capacity and Coulombic efficiency for the first 100 galvanostatic cycles of the phosphorene/graphene (48.3 wt% P)
anode tested under different currents.
and 10 C (26 A g−1), the capacity retentions were 84 and 77%,
respectively, after 100 cycles. SEI breakdown and reformation
usually results in a poor Coulombic efficiency, an indicator of
the reversibility of the electrode reaction. At the relatively fast rates
of 3 C (8 A g−1) and 10 C (26 A g−1), the average Coulombic efficiencies were 97.6 and 99.3%, respectively, in the first 100 cycles. With
an increase of mass loading from 0.5 to 2 mg cm−2, despite the
fact that specific capacities decrease by 5–12% at various C rates,
the phosphorene–graphene structure still exhibited a high capacity
retention of 88% after 70 cycles at a rate of 0.7 C (1.8 A g−1)
(Supplementary Fig. 12), an indication of the stability of the
hybrid material.
Conclusions
We have reported a sandwiched phosphorene–graphene hybrid
material that shows high specific capacity, rate capability and
cycle life as an anode material in sodium-ion batteries. The graphene layers provide an elastic buffer layer to accommodate the
expansion of phosphorene layers along the y and z axial directions
during the alloy reaction to Na3P. The phosphorene nanosheets,
with increased interlayer distance, offer a short and effective diffusion distance for sodium ions. Moreover, the graphene enhances
both the electrical conductivity of the material and provides a preferential pathway to the electrons generated by the redox reaction
of phosphorene. The facile synthesis and superior electrochemical
properties of our sandwiched phosphorene–graphene hybrid
could render it a suitable anode material for sodium-ion batteries.
Methods
Methods and any associated references are available in the online
version of the paper.
Received 30 October 2014; accepted 27 July 2015;
published online 7 September 2015
References
1. Slater, M. D., Kim, D., Lee, E. & Johnson, C. S. Sodium-ion batteries. Adv. Funct.
Mater. 23, 947–958 (2013).
2. Barpanda, P., Oyama, G., Nishimura, S.-I., Chung, S.-C. & Yamada, A. A 3.8-V
earth-abundant sodium battery electrode. Nature Commun. 5, 4358 (2014).
3. Ellis, B. L., Makahnouk, W. R. M., Makimura, Y., Toghill, K. & Nazar, L. F. A
multifunctional 3.5 V iron-based phosphate cathode for rechargeable batteries.
Nature Mater. 6, 749–753 (2007).
4. Lee, H.-W. et al. Manganese hexacyanomanganate open framework as a highcapacity positive electrode material for sodium-ion batteries. Nature Commun.
5, 5280 (2014).
5. Palomares, V. et al. Na-ion batteries, recent advances and present challenges
to become low cost energy storage systems. Energy Environ. Sci. 5,
5884–5901 (2012).
6. Komaba, S. et al. Electrochemical Na insertion and solid electrolyte interphase
for hard-carbon electrodes and application to Na-ion batteries. Adv. Funct.
Mater. 21, 3859–3867 (2011).
7. Cao, Y. L. et al. Sodium ion insertion in hollow carbon nanowires for battery
applications. Nano Lett. 12, 3783–3787 (2012).
8. Chevrier, V. L. & Ceder, G. Challenges for Na-ion negative electrodes.
J. Electrochem. Soc. 158, A1011–A1014 (2011).
9. Abel, P. R. et al. Nanocolumnar germanium thin films as a high-rate sodium-ion
battery anode material. J. Phys. Chem. C 117, 18885–18890 (2013).
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
5
ARTICLES
NATURE NANOTECHNOLOGY
10. Baggetto, L., Keum, J. K., Browning, J. F. & Veith, G. M. Germanium as
negative electrode material for sodium-ion batteries. Electrochem. Commun. 34,
41–44 (2013).
11. Zhu, H. et al. Tin anode for sodium-ion batteries using natural wood fiber as a
mechanical buffer and electrolyte reservoir. Nano Lett. 13, 3093–3100 (2013).
12. Wang, J. W., Liu, X. H., Mao, S. X. & Huang, J. Y. Microstructural evolution
of tin nanoparticles during in situ sodium insertion and extraction. Nano Lett.
12, 5897–5902 (2012).
13. Jow, T. R., Shacklette, L. W., Maxfield, M. & Vernick, D. The role of conductive
polymers in alkali-metal secondary electrodes. J. Electrochem. Soc. 134,
1730–1733 (1987).
14. Darwiche, A. et al. Better cycling performances of bulk Sb in Na-ion batteries
compared to Li-ion systems: an unexpected electrochemical mechanism. J. Am.
Chem. Soc. 134, 20805–20811 (2012).
15. Corbridge, D. E. C. Phosphorus Chemistry, Biochemistry and Technology 6th edn
(CRC Press, 2013).
16. Cotton, A., Wilkinson, G., Murillo, C. A. & Bochmann, M. Advanced Inorganic
Chemistry (Wiley, 1999).
17. Qian, J., Qiao, D., Ai, X., Cao, Y. & Yang, H. Reversible 3-Li storage reactions of
amorphous phosphorus as high capacity and cycling-stable anodes for Li-ion
batteries. Chem. Commun. 48, 8931–8933 (2012).
18. Wang, L. et al. Nano-structured phosphorus composite as high-capacity anode
materials for lithium batteries. Angew. Chem. Int. Ed. 51, 9034–9037 (2012).
19. Marino, C., Debenedetti, A., Fraisse, B., Favier, F. & Monconduit, L. Activatedphosphorus as new electrode material for Li-ion batteries. Electrochem.
Commun. 13, 346–349 (2011).
20. Li, W.-J., Chou, S.-L., Wang, J.-Z., Liu, H.-K. & Dou, S.-X. Simply mixed
commercial red phosphorus and carbon nanotube composite with exceptionally
reversible sodium-ion storage. Nano Lett. 13, 5480–5484 (2013).
21. Qian, J., Wu, X., Cao, Y., Ai, X. & Yang, H. High capacity and rate capability of
amorphous phosphorus for sodium ion batteries. Angew. Chem. Int. Ed. 125,
4731–4734 (2013).
22. Kim, Y. et al. An amorphous red phosphorus/carbon composite as a promising
anode material for sodium ion batteries. Adv. Mater. 25, 3045–3049 (2013).
23. Yabuuchi, N. et al. Phosphorus electrodes in sodium cells: small volume
expansion by sodiation and the surface-stabilization mechanism in aprotic
solvent. ChemElectroChem 1, 580–589 (2014).
24. Morita, A. Semiconducting black phosphorus. Appl. Phys. A 39, 227–242 (1986).
25. Hultgren, R., Gingrich, N. S. & Warren, B. E. The atomic distribution in red and
black phosphorus and the crystal structure of black phosphorus. J. Chem. Phys.
3, 351–355 (1935).
26. Nagao, M., Hayashi, A. & Tatsumisago, M. All-solid-state lithium secondary
batteries with high capacity using black phosphorus negative electrode. J. Power
Sources 196, 6902–6905 (2011).
27. Park, C.-M. & Sohn, H.-J. Black phosphorus and its composite for lithium
rechargeable batteries. Adv. Mater. 19, 2465–2468 (2007).
28. Sun, L.-Q. et al. Electrochemical activity of black phosphorus as an anode
material for lithium-ion batteries. J. Phys. Chem. C 116, 14772–14779 (2012).
29. Sun, J. et al. Formation of stable phosphorus–carbon bond for enhanced
performance in black phosphorus nanoparticle–graphite composite battery
anodes. Nano Lett. 14, 4573–4580 (2014).
30. Ramireddy, T. et al. Phosphorus–carbon nanocomposite anodes for lithium-ion
and sodium-ion batteries. J. Mater. Chem. A 3, 5572–5584 (2015).
31. Yazami, R. & Universitaire, D. A reversible graphite–lithium electrochemical
generators. J. Power Sources 9, 365–371 (1983).
6
DOI: 10.1038/NNANO.2015.194
32. Wu, H. et al. Stable cycling of double-walled silicon nanotube battery
anodes through solid-electrolyte interphase control. Nature Nanotech. 7,
310–315 (2012).
33. Liu, N. et al. A pomegranate-inspired nanoscale design for large-volume-change
lithium battery anodes. Nature Nanotech. 9, 187–192 (2014).
34. Li, W. et al. High-performance hollow sulfur nanostructured battery cathode
through a scalable, room temperature, one-step, bottom-up approach. Proc. Natl
Acad. Sci. USA 110, 7148–7153 (2013).
35. Ye, P. D. et al. Phosphorene: an unexplored 2D semiconductor with a high hole
mobility. ACS Nano 8, 4033–4041 (2014).
36. Reich, E. S. Phosphorene excites materials scientists. Nature 506, 19 (2014).
37. Churchill, H. O. H. & Jarillo-Herrero, P. Two-dimensional crystals: phosphorus
joins the family. Nature Nanotech. 9, 330–331 (2014).
38. Li, L. et al. Black phosphorus field-effect transistors. Nature Nanotech. 9,
372–377 (2014).
39. Mohiuddin, T. et al. Uniaxial strain in graphene by Raman spectroscopy: G
peak splitting, Grüneisen parameters, and sample orientation. Phys. Rev. B
79, 205433 (2009).
40. Ferrari, A. C. et al. Raman spectrum of graphene and graphene layers. Phys. Rev.
Lett. 97, 187401 (2006).
41. Brent, J. R. et al. Production of few-layer phosphorene by liquid exfoliation of
black phosphorus. Chem. Commun. 50, 13338–13341 (2014).
42. Hernandez, Y. et al. High-yield production of graphene by liquid-phase
exfoliation of graphite. Nature Nanotech. 3, 563–568 (2008).
43. Sugai, S. Raman and infrared reflection spectroscopy in black phosphorus.
Solid State Commun. 53, 753–755 (1985).
44. Vanderborgh, C. A. & Schiferl, D. Raman studies of black phosphorus from
0.25 to 7.7 GPa at 15 K. Phys. Rev. B 40, 9595–9599 (1989).
45. Akahama, Y., Kobayashi, M. & Kawamura, H. Raman study of black phosphorus
up to 13 GPa. Solid State Commun. 104, 311–315 (1997).
46. Lu, W. et al. Plasma-assisted fabrication of monolayer phosphorene and its
Raman characterization. Nano Res. 7, 853–859 (2014).
Acknowledgements
This work was supported by the Department of Energy, Office of Basic Energy Sciences,
Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515.
H.W.L. acknowledges support from the Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of Education,
Science and Technology under NRF-2012R1A6A3A03038593.
Author contributions
Y.C. and J.S. conceived and designed the experiments. J.S. performed sample fabrication,
characterization and electrochemical measurements. H.W.L. participated in part of the
experiments and conducted in situ TEM and HR-TEM characterization. J.S., M.P., H.W.L.
and Y.C. co-wrote the paper. All authors discussed the results and commented on the
manuscript. J.S. and H.W.L. contributed equally to this work.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints. Correspondence and
requests for materials should be addressed to Y.C.
Competing financial interests
The authors declare no competing financial interests.
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
NATURE NANOTECHNOLOGY
DOI: 10.1038/NNANO.2015.194
Methods
Phosphorene and graphene synthesis. Both phosphorene and graphene were
prepared by a liquid-phase exfoliation method adapted from the procedure reported
previously41,42. Black phosphorus was dispersed in NMP (cylindrical vial, 20 ml
NMP) at a concentration of 0.1 mg ml–1 and sonicated in a sonic
bath (Branson 5210 Ultrasonic) for 10 h. Graphite was also dispersed in NMP
(20 ml NMP) at a concentration of 0.1 mg ml–1 and sonicated in a sonic bath for
30 min. The resultant dispersion of graphene was centrifuged using a Fisher
Scientific accuSpin 400 centrifuge for 60 min at 500 r.p.m. to remove sediment and
obtain the supernatant, while the resultant dispersion of phosphorene was
centrifuged at 5,000 r.p.m. for 30 min. After centrifuging, decantation was carried
out by pipetting off the top half of the dispersion. These methods resulted in
phosphorene and graphene yields (mass of phosphorene or graphene/starting
material mass) of 17 and 3 wt%, respectively. The sediments of black phosphorus
and graphite could be reused to produce phosphorene and graphene by repeating the
above processes.
Phosphorene–graphene synthesis. Dispersions of phosphorene and graphene were
mixed evenly in different volume ratios by stirring, to achieve different carbon/
phosphorus molar ratios (Supplementary Table 2). Because phosphorene is oxidized
slowly in air, the sandwiched phosphorene–graphene was produced via selfassembly by evaporating the dispersion at 120 °C in a vacuum oven or an
ARTICLES
argon-filled glove box. The sample was washed with anhydrous ethyl alcohol several
times to remove residual NMP, and then heated at 80 °C to evaporate ethyl alcohol to
obtain a powder.
Electrochemical characterization. Battery performance was evaluated by
galvanostatic cycling of coin (CR 2032) cells with the phosphorene–graphene
composite as the working electrode and sodium foil as the counter/reference
electrode. The working electrodes were made using a typical slurry method with
phosphorene–graphene powders and polyvinylidene fluoride (PVDF) binder with a
mass ratio of 9:1 in N-methyl-pyrrolidone (NMP) solvent. The mass loading of
active material (phosphorene) was ∼0.5 mg cm−2, corresponding to a total mass
loading of ∼1.15 mg cm−2. The thickness of the electrode without a current collector
was 6.5 ± 0.3 μm. The electrolyte consisted of 1.0 mol l−1 NaPF6 in an ethylene
carbonate (EC)–diethyl carbonate (DEC) solution to which was added 10%
fluoroethylene carbonate (FEC), which effectively forms stable SEI layers for
sodium-ion batteries. Electrochemical data were collected using an Arbin MSTAT
battery test system within the potential range 0.02–1.5 V (versus Na/Na+) at various
current densities. The specific capacity was calculated based on the weight of
phosphorus. The mass loading of active material was increased to 2 mg cm−2 for
further study of its performance. The thickness of the electrode without a current
collector was 23.8 ± 0.4 μm. The rate and cycling performance are presented in
Supplementary Fig. 12.
NATURE NANOTECHNOLOGY | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved